Aluminum alloys containing indium and/or zinc are used commercially as sacrificial galvanic anodes for protecting ferrous metals from electrolytic attack. Such alloys, containing indium and/or zinc, are disclosed in, e.g., U.S. Pat. No. 3,172,760; U.S. Pat. No. 3,418,230; U.S. Pat. No. 1,997,165; U.S. Pat. No. 3,227,644; U.S. Pat. No. 3,312,545; U.S. Pat. No. 3,616,420; U.S. Pat. No. 2,023,512; and U.S. Pat. No. 2,565,544.
In the December, 1966 issue of Materials Protection there are two publications which contain teachings of Al-In-Zn alloys for use as galvanic anodes. One publication is entitled "The Influence of Alloying Elements on Aluminum Anodes in Sea Water", pp. 15-18. The other publication is entitled "Tests on the Effects of Indium for High Performance Aluminum Anodes", pp. 45-50. These publications imply, as do various patents named above, that best results are obtained by the use of high purity aluminum in the Al-In-Zn alloys and that impurities in the aluminum are detrimental unless properly controlled.
U.S. Pat. No. 3,496,085 pertains to an aluminum anode containing minor amounts of mercury and zinc in which silicon is present in an amount in excess of the normal impurity level. The amounts of silicon and iron are controlled within certain ranges and ratios.
It is well known that the principal impurities normally found in aluminum are iron, silicon, and copper. It is generally felt by practitioners of the galvanic anode art, that best results are attained by holding the amount of these naturally occurring impurities to a very low level of concentration. It is generally believed that anodes prepared from high purity aluminum (about 99.99% purity) give better performance than anodes prepared from commercial grade aluminum (about 99.8 to about 99.9% purity).